I'm looking at forming a roof for my boxcab electric in 1" scale ride on railroading.

I've attached the following diagram. I have several thoughts on how to do this.

I will say that I am OK with having ribs welded to the roof, for structure/rigidity.

For that reason, one option is to calculate the arc length of the 30 inch radius, then position my 3/4" radii accordingly. Then form/force the roof to the ribs, and tack weld them in place. This might be something I can do with minimal tooling. Perhaps a piece of bar clamped to the table.

I'm not sure what my other options are, yet. Thoughts? I have access to a sheet metal brake if needed.

Assuming that your brake can handle the gauge AND if the bending edge can be spaced backwards a bit...you can put a half or quarter round on the bending edge. If the gauge isn't that hefty, you should be able to use wood - try it on a piece of scrap first.

I've used the method Russ described above (it works fine), but instead of a rod connected to a 2x4 I took a piece of MDF and routed the edge. To my surprise the MDF held up fine. It was very easy to radius the edge with the router and very smooth.

1. I rounded the 1/2" thick MDF with a standard 1/2" round over bit in a router. Cuts like butter and very smooth.
2. Put the rounded edge of the MDF face down on top of the metal to be bent. Aligned edge, etc.

3. Bend an usual. (works the same in a DIY brake). Over-bend as required.